1. Field of the Invention
The present invention relates to an image forming apparatus for forming a toner image on an image carrier with an electrophotographic process and transferring the toner image to a sheet or recording medium either directly or via an intermediate image transfer body. Also, the present invention relates to a developing device included in an image forming apparatus and using a plurality of developing rollers arranged side by side in the direction of rotation of an image carrier and operable with a two-ingredient type developer, i.e., a toner and carrier mixture, and a cleaning device also included in the image forming apparatus for removing residual toner and impurities left on the image carrier with a cleaning blade.
2. Description of the Background Art
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus to charge and then scan an image carrier imagewise for thereby forming a latent image and develop the latent image with toner. The resulting toner image is transferred to a sheet or recording medium and then fixed on the sheet.
Toner with a small grain size enhances image quality, but is defectively charged due to an increase in the carrier coating ratio of the toner, as known in the art. To solve this problem, it has been customary to increase the surface area of the individual carrier grain for a unit weight for thereby reducing the carrier coating ratio of the toner, so that the probability that the toner contacts the carrier increases. However, another problem with toner having a small grain size is that the carrier easily deposits on the image carrier.
On the other hand, considering the increasing demand for high-speed image formation, developing ability available with a single developing roller is short. In light of this, a plurality of developing rollers may be used. However, if the diameter of each developing roller is reduced to meet the demand for the size reduction of an image forming apparatus, then the rotation speed of the developing roller and therefore a centrifugal force to act on the carrier increases, aggravating carrier deposition on the image carrier. The carrier deposited on the image carrier damages the edge of a cleaning blade expected to remove residual toner from the image carrier. Further, if such carrier is transferred from the image carrier to a sheet, then it damages the surfaces of a pair of fixing rollers when being conveyed via the nip of the fixing rollers. In this manner, the carrier deposited on the image carrier degrades the reliability of the image forming apparatus. Moreover, the carrier deposited on the image carrier increases image density on a sheet and thereby smears an image, lowering image quality.
To obviate carrier deposition stated above, Japanese Patent Laid-Open Publication No. 6-51628 and Japanese Patent No. 2,930,812, for example, each define a specific linear velocity of an image carrier and that of a developing roller by using a magnet roller whose magnetic force is weak. However, developing ability available with the above documents is short because use is made of only one developing roller.
Further, to obviate carrier deposition, the flux density of a main pole for development included in a developing roller may be increased, as proposed in the past. This scheme is directed mainly toward a developing device of the type using a single developing roller. If such a scheme is applied to a developing device of the type using a plurality of developing rollers, then it effects the flow of a developer between the developing rollers and causes an excessive amount of developer to be conveyed, resulting in overflow and other troubles.
For example, assume that two developing rollers are positioned side by side in the direction of rotation of the image carrier, and that the flux density of the main pole of the downstream developing roller is increased. Then, such an intense magnetic force scoops up the developer even via the gap between the two developing rollers with the result that an excessive amount of developer deposits on the rollers and brings about various problems including the smearing of an image.
Further, the intense magnetic force of the main pole intensifies even a magnetic force at the rear of the main pole, preventing the developer from parting from the downstream developing roller. Consequently, the developer moves in accordance with the rotation of the downstream developing roller and again reaches the upstream developing roller. This is also apt to bring about the smearing of an image and the overflow of the developer. It is therefore difficult to obviate carrier deposition on the image carrier by intensifying the magnetic force.
To increase the magnetic force of the main pole of the developing roller, use may be made of a rare earth magnet exerting an intense magnetic force, as known in the art. Such a magnet, however, intensifies the magnetic forces of the other poles as well and therefore makes it difficult to establish optimum balance between the poles while increasing cost.
To establish optimum balance between poles, we prepared a cylindrical magnet roller by combining magnets each forming a particular pole and used a rare earth magnet for one of the magnets forming the main pole. However, the rare earth magnet with an intense magnetic force caused a developer to follow the rotation of a developing roller and overflow.
Japanese Patent No. 2,545,601 and Japanese Patent Laid-Open Publication No. 2000-81789, for example, each also propose a cylindrical magnet roller in which a rare earth magnet is buried at the main pole for development. However, in U.S. Pat. No. 2,545,601, the rare earth magnet is 1.15 mm long or thick in the radial direction of the roller and 5 mm long or wide in the circumferential direction of the roller. In this case, although the length in the radial direction is small, the length in the circumferential direction is great and causes the intense magnetic force to effect the other poles, again resulting in the problem stated above. Conversely, in Laid-Open Publication No. 2000-81789, although the rare earth magnet is as short or thin as 3 mm in the radial direction of the roller, it is as long or wide as 4 mm in the circumferential direction of the roller, also resulting in the above problem.
On the other hand, after the transfer of a toner image from the image carrier to a sheet, some toner is left on the image carrier as residual toner. It is therefore a common practice to remove, before the formation a new latent image, the residual toner as well as impurities including paper dust and rosin, Mg, Al, K, and other additives contained in a sheet from the image carrier. Such additives are contained not only in a sheet but also in toner for implementing various characteristics, including chargeability, fixability and fluidity, required of toner.
To remove the residual toner and impurities left on the image carrier, use is often made of a cleaning blade formed of polyurethane or similar elastic material and having its edge pressed against the surface of the image carrier by preselected pressure. However, a problem with the cleaning blade is that as cleaning is repeated, the toner and impurities tend to accumulate between the image carrier and the cleaning blade and vary the pressing condition of the blade, preventing the expected cleaning effect from being achieved. The toner and impurities so caught between the image carrier and the cleaning blade sometimes include even masses of toner. Consequently, if such toner and impurities get through the cleaning blade, cleaning efficiency is lowered and brings about defective images ascribable to the background contamination of the image carrier. In this connection, when a mass of toner is caught between the image carrier and the cleaning blade, the residual toner on the image carrier gets through the cleaning blade at both sides of the mass.
On the other hand, while the cleaning blade is pressed against the image carrier with preselected pressure, high pressure acts at a portion where the edge of the cleaning blade contacts the image carrier only over a small area. In this condition, if cleaning is repeated with the toner and impurities being caught by the edge of the cleaning blade, then the toner and impurities damage the surface of the image carrier or cause the toner, pressed against the image carrier, to form a thin layer on the image carrier (so-called toner filming). As a result, photoelectric characteristics, particularly chargeability, is lowered on the surface of the image carrier, resulting in low image quality.
In light of the above, when the image carrier is brought to a stop after image formation, the pressure of the cleaning blade acting on the image carrier may be lowered or canceled while the image carrier may be moved in the reverse direction, as proposed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-155514 (column “0030, FIG. 5) and 05-119687 (column “0011”, FIG. 1) Further, the image carrier may be again moved in the forward direction after the reverse movement and then stopped, as taught in, e.g., Japanese Patent Laid-Open Publication No. 07-175394. In any case, the reverse rotation of the image carrier is used to cancel pressure acting on the toner and impurities caught by the edge of the cleaning blade, thereby promoting the removal.
However, the conventional reverse rotation schemes stated above have the following problems left unsolved. The cleaning device taught in, e.g., Laid-Open Publication No. 05-119687 mentioned earlier includes a seal positioned at the inlet of the cleaning device where toner is apt to drop and smear surrounding. As for this type of cleaning device, when the image carrier is moved in the reverse direction, it is possible to efficiently remove the impurities caught by the edge of the cleaning blade by increasing the amount of reverse movement in both of the configuration of Laid-Open Publication No. 2000-155514 lacking a seal and the configuration of Laid-Open Publication No. 05-119687 including a seal. However, the portion of the image carrier facing the cleaning device is sometimes moved over the inlet of the cleaning device with the result that the toner deposited on part of the image carrier moved over the inlet drops due to gravity or friction acting between it and the seal.
Particularly, when a peeler or similar sheet separating member and an image density sensor are positioned around the cleaning device, the toner thus dropped from the image carrier accumulates on such members and therefore smears sheets or renders the output of the image density sensor erroneous. More specifically, the toner deposited on the peeler varies frictional resistance between the peeler and a sheet to thereby bring about defective sheet separation or smears the sheet. Also, the toner deposited on the image density sensor makes the output of the sensor differ from the actual image density. Moreover, toner deposits more on the portion of the image carrier facing the cleaning member than on the other portion of the drum. Therefore, when the portion facing the cleaning member moves over the inlet of the cleaning device, a large amount of toner drops and makes the above problem more serious.
Generally, the toner left on the image carrier can be removed more easily if the surface of the image carrier has a smaller coefficient of friction. Stated another way, the removal efficiency decreases with an increase in the coefficient of friction. Particularly, we experimentally found that cleaning ability was lowered when the coefficient of friction was 0.2 or below, as described in copending U.S. patent application Ser. No. 10/418,111 filed on Apr. 18, 2003.
The coefficient of friction has influence on friction energy acting between the cleaning blade and the image carrier. If friction energy is high, then toner is apt to melt and adhere to the image carrier and thereby degrade the removal efficiency of the cleaning blade. This is particularly true when toner with a small grain size is used for enhancing resolution, because such toner has small thermal capacity. The toner adhered to the image carrier and unable to be removed brings about filming stated earlier and deteriorates characteristics on the surface of the image carrier throughout the consecutive image forming steps.
Filming is effected by the hardness of the surface of the image carrier as well. More specifically, when surface hardness is low, the cleaning blade grinds the surface of the image carrier and refreshes it, so that filming occurs little. However, when the image carrier is formed of amorphous silicone (a-Si) implementing a hard surface that wears little or is provided with a surface layer containing inorganic grains, it is difficult for the cleaning blade to grind the surface and therefore obviate filming.
In the configuration wherein the image carrier is moved in the reverse direction for the purpose stated earlier, the cleaning blade is caused to warp in the opposite direction by the image carrier moving in the reverse direction, allowing the toner and impurities to be released from the edge of the cleaning blade. However, when the image carrier is again moved in the forward direction, it is likely that the toner and impurities so released are again caught by the edge of the cleaning blade. It is therefore difficult to fully prevent the cleaning blade from catching the toner and impurities. This is particularly true when the cleaning blade contacts the image carrier at the downstream edge of its end face, as determined by experiments.